The law firm Carter-Ruck has made a fresh move that could stop an MPs' debate next week by claiming a controversial injunction it has obtained is "sub judice".

The move follows the revelation of the existence of a secret "super-injunction" obtained by the firm on behalf of the London-based oil traders Trafigura.

The injunction not only bans disclosure of a confidential report on Trafigura and toxic waste, but also banned disclosure of the injunction's very existence, until it was revealed by an MP this week under parliamentary privilege.

Carter-Ruck partner Adam Tudor today sent a letter to the Speaker, John Bercow, and also circulated it to every single MP and peer, saying they believed the case was "sub judice".

If correct, it would mean that, under Westminster rules to prevent clashes between parliament and the courts, a debate planned for next Wednesday could not go ahead.

Earlier this week, the Labour MP Paul Farrelly said Carter-Ruck might be in contempt of parliament for seeking to stop the Guardian reporting questions he had put down on the order paper revealing the existence of the "super-injunction".

The Conservative MP Peter Bottomley went on to tell Gordon Brown at prime minister's questions that he would report Carter-Ruck to the Law Society for obtaining an injunction that purported to ban parliamentary reporting.

Carter-Ruck said in a letter and press release that, although the Speaker had discretion over sub judice questions, "we believe the proceedings to have been and to remain 'active' within the definition of House Resolution ... of 15 November 2001 in that arrangements have been made for the hearing of an application before the Court".

Bercow had told MPs the previous day: "It is not sub judice under the house's rules ... There is no question of our own proceedings being in any way inhibited."

Evan Harris, the Liberal Democrat MP who secured next week's debate, said: "I read with interest the letter from Carter-Ruck. I do not think that sub judice is involved here and I do not think that MPs will be deterred from discussing this case in the debate without a ruling from the Speaker, which he has not as yet indicated any likelihood of providing."

Farrelly said: "Carter-Ruck's manoeuvres this week, were it not so serious, would be tantamount to high farce. It is important MPs should not be prevented from going ahead with debates next week."

The prominent media lawyer Mark Stephens said: "This sort of assault on democratic privileges is what you would expect to see in a banana republic."